As an adaptive response to starvation, the hypothalamic- pituitary-thyroid axis is down-regulated. This is caused, at least in part, by suppression of proTRH mRNA expression in the hypothalamus, which can be reversed by leptin. Some data suggest that the action of leptin on TRH neurons in the paraventricular nucleus (PVN) of the hypothalamus occur through an indirect pathway involving the arcuate nucleus (AC) of the hypothalamus. This may involve release of neuropeptides such as NPY, MSH AgRP from AC neurons that are leptin responsive and that project to the PVN where the hypophysiotropic neurons producing proTRH are located. However, a new line of work done recently in our laboratories strongly suggests that TRH neurons may also be regulated directly by leptin without intermediate pathways. Thus, in this proposal we will identify molecular events involved in the action of leptin on the proTRH life cycle in TRH neurons. As a model system we will utilize our established primary cultures of hypothalamic neurons that express high levels of endogenous proTRH and leptin receptors. Some aspects will be corroborated with in vivo studies.